1. Field of the Invention
The present invention relates to a focus servo apparatus.
2. Description of the Related Art
Currently, an optical disc apparatus for reproducing or recording information from or into an optical disc (CD (Compact Disc), DVD (Digital Versatile Disc), etc.) is widely used. While reproducing or recording information from or into an optical disc, the optical disc apparatus performs focus servo to move an objective lens that converges laser beam, in a focusing direction orthogonal to an information recording surface, in order to focus laser beam on the information recording surface of the optical disc in response to the wobbling or the like thereof. In order to set the target position of the objective lens in the focusing direction when performing the focus servo, the optical disc apparatus executes the so-called focus search to detect the objective lens position at which laser beam is focused on the information recording surface, as an initial operation based on the turning-on of a power supply voltage, for example.
The focus search is described in detail hereinafter with reference to FIGS. 5 to 7. FIG. 5 is a block diagram briefly illustrating the configuration of an optical disc apparatus 100. FIG. 6 is a waveform diagram showing a focus error signal (hereinafter, referred to as FE signal) and an output signal of a focus servo apparatus 102 (FDO). FIG. 7 is a diagram showing the movement of an objective lens 105 included in an optical pickup 104 in the focusing direction according to the level of the output signal FDO. Description is made supposing that the position of the objective lens 105 before the start of focus search is a position drawn by a thick solid line in FIG. 7. The above position of the objective lens 105 before the start of focus search means: a position corresponding to the level of an output voltage FDO (VREF) when the power supply voltage is turned on for the optical disc apparatus 100, for example; or a position corresponding to the level of an output voltage FDO (VREF) at the end of the previous focus servo operation.
The optical disc apparatus 100 enters the focus search mode when the power supply is turned-on. A microcomputer 101 transmits to the focus servo apparatus 102 a start signal for executing a process (fruitless search) of detecting the top level (S_MAX) and the bottom level (S_MIN) of the FE signal in the focus search, in order to correspond to different reflectance for each type of an optical disc 120. First, the focus servo apparatus 102 outputs the output signal FDO at a level corresponding to a predetermined solid-line position (FDOFFSETX) in order to move the objective lens 105 in the focusing direction to the predetermined solid-line position (t10). The predetermined solid-line position is generally determined to be set in the middle of the movable range (between a dot-dashed line position and a double-dot-dashed line position) of the objective lens 105 in the focusing direction, which is determined depending on the characteristics of a focus actuator 106 (magnetic action between a magnetic member and a focus servo coil, etc.) or the like. Because, in the conventional focus search, the fruitless search and the process (focus-on search) of detecting the level of an output signal FDO at which laser beam converged by the objective lens 105 is focused on (hereinafter, referred to as focus-on) the information recording surface of the optical disc 120, are performed, while the objective lens 105 moves from the dot-dashed line position to the double-dot-dashed line position (or from the double-dot-dashed line position to the dot-dashed line position), according to a mode instructed by the microcomputer 101. Therefore, in order to correspond promptly to any mode, the focus servo apparatus 102 performs the fruitless search and the focus-on search from the predetermined solid-line position as an initial position for the start of the focus search. The objective lens 105 is moved to the predetermined solid-line position by the focus actuator 106 that is applied with a control voltage corresponding to the output signal FDO at FDOFFSETX from a driver 103.
Subsequently, the focus servo apparatus 102 decreases the level of the output signal FDO to FDOFFSETX−FSTOP (t10 to t11) in order to increase the level of the output signal FDO from, e.g., FDOFFSETX−FSTOP to FDOFFSETX+FSTOP for the detection of the top and the bottom levels of an FE signal. Thereby, the objective lens 105 moves to the dot-dashed line position corresponding to the output signal FDO at FDOFFSETX−FSTOP. The focus servo apparatus 102 outputs an output signal FDO increasing in level from FDOFFSETX−FSTOP to FDOFFSETX+FSTOP (t11 to t12). Thereby, the objective lens 105 moves from the dot-dashed line position corresponding to the output signal FDO at FDOFFSETX−FSTOP to the double-dot-dashed line position corresponding to the output signal FDO at FDOFFSETX+FSTOP. During the focus search, the optical pickup 104 emits the laser beam with a wave length conformed to the standard of the optical disc 120 (CD: 780 nm to 790 nm, DVD: 650 nm to 660 nm, etc.). The laser beam is converged by the objective lens 105 to be emitted to the optical disc 120. The optical pickup 104 then generates a photoelectric conversion signal from the reflected light of the laser beam applied to the information recording surface of the optical disc 120, and outputs the photoelectric conversion signal to an RF (Radio Frequency) amplifier 107. The RF amplifier 107 generates an FE signal, an RF signal, etc. based on the photoelectric conversion signal, and outputs the FE signal to the focus servo apparatus 102. The focus servo apparatus 102 detects the top and the bottom levels of the FE signal, and detects: whether the top level of the FE signal does not reach +FS_TH (the top level thereof in absolute value is smaller than +FS_TH in absolute value); and whether the bottom level of the FE signal does not reach −FS_TH (the bottom level thereof in absolute value is smaller than −FS_TH in absolute value). The focus servo apparatus 102 so adjusts the gain of a variable gain amplifier circuit (not shown) included in the focus servo apparatus 102 for amplifying the FE signal, as to increase the top level up to greater than or equal to +FS_TH, if the focus servo apparatus 102 detects that, for example, the top level of the FE signal does not reach +FS_TH. The focus servo apparatus 102 then decreases the level of the output signal FDO to FDOFFSETX in order to end the fruitless search (t12 to t13). Thus the objective lens 105 moves to the solid-line position that is predetermined based on the output signal FDO at FDOFFSETX (end of fruitless search).
Subsequently, the microcomputer 101 transmits a signal for performing focus-on search to the focus servo apparatus 102. The focus servo apparatus 102 first decreases the level of the output signal FDO to FDOFFSETX−FSTOP (t14 to t15) in order to increase the level of the output signal FDO, e.g., from FDOFFSETX−FSTOP to FDOFFSETX+FSTOP for the detection of the level of the output signal FDO at focus-on. Thus the objective lens 105 moves to the dot-dashed line position corresponding to the output signal FDO at FDOFFSETX−FSTOP. The focus servo apparatus 102 outputs an output signal FDO which increases in level from FDOFFSETX−FSTOP to FDOFFSETX+FSTOP (t15 to t17). The focus servo apparatus 102 then detects whether the FE signal has reached +FS_TH in the process of increasing the level of the output signal FDO from FDOFFSETX−FSTOP. If the focus servo apparatus 102 detects that the FE signal has reached +FS_TH (t16), the level of the output signal FDO when the FE signal returns to “0” is detected. If the focus servo apparatus 102 detects that the FE signal has returned to “0” (t17), it is then detected that the level of the output signal FDO (FDO(n)) is an level at which the focus-on can be performed(end of focus-on search and focus search).
The focus servo apparatus 102 sets the level of the output signal FDO at the start of the focus servo at the FDO(n). Consequently, the focus servo is started in order to reproduce or record information from or into the optical disc 120, while the objective lens 105 being maintained at the state of being positioned at a broken-line position based on the output signal FDO at the level of FDO(n) (t17 or later). During a period when information is reproduced or recorded from or into the optical disc 120, laser beam is focused on the information recording surface of the optical disc 120 as a result of performing focus servo based on the output signal FDO at a level obtained by adding or subtracting a level according to the FE signal to or from the FDO(n). It is thereby achieved that information is reproduced or recorded in satisfactory condition (see Japanese Utility Model Laid-Open Publication No. 5-55310, and Japanese Patent No. 3476112).
However, in a conventional focus servo apparatus, there has been a possibility of an abrupt change in the level of an output signal FDO since the level of the output signal FDO (VREF) before the start of focus search is set at FDOFFSETX in order to move the objective lens 105 to the predetermined solid-line position. In this case, control voltage applied to the focus actuator 106 changes abruptly, and thus there has been a possibility that the objective lens 105 vibrates. If the objective lens 105 vibrates, there has been a possibility that the focus actuator 106 cannot respond to a control voltage subsequently applied thereto, which results in a risk that focus search cannot be performed in a satisfactory condition. Or there has been a risk that focus search needs a longer time.